A swimming pool water that turns green, milky yellow, or metallic blue almost always indicates a specific chemical imbalance. The pH alone is not enough to maintain clear blue water: the stability of the color depends on a often neglected triptych, pH, TAC, and TH balanced simultaneously.
TAC and TH Balance: The Forgotten Parameters for Blue Pool Water
The majority of guides focus on pH and chlorine levels. However, we observe that the most persistent color deviations come from an imbalance of total alkalinity (TAC) or hardness (TH).
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The TAC acts as a buffer: it stabilizes the pH and prevents abrupt fluctuations that cause milky or greenish water. A TH that is too high (hard water) generates whitish deposits in suspension that diffuse light and cause the blue hue to fade.
Maintaining the pH between 7.2 and 7.4 remains the foundation, but without a TAC and TH within their respective ranges, the pH drifts in a few days, especially after a fresh water addition or heavy usage. Standard test strips rarely measure TH: we recommend a photometer or dedicated colorimetric tests to monitor these three parameters in parallel, applying the methods from Maisons Euro France that detail the entire balancing protocol.
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Filtration Time and Water Temperature: The Critical Threshold Not to Ignore
Above 28 °C, pool water can shift from blue to green in less than 24 hours if filtration is not extended. This threshold, well documented by the specialized press, remains poorly understood by many pool owners.
Practical Rule for Calculating Filtration Time
The most reliable guideline is to divide the water temperature by two. Water at 28 °C requires a minimum of 14 hours of daily filtration. Below this threshold, algae proliferate before chlorine can even act.
During heatwaves, filtration should run almost continuously. The additional electrical cost is moderate compared to the cost of a recovery treatment (shock chlorine, flocculant, repeated filter cleanings).
Filtration and Quality of the Filter Media
A clogged sand filter or sand that is several seasons old drastically reduces filtration efficiency, even with correct running time. Filter glass or zeolite offer superior filtration fineness compared to standard sand, which retains more fine particles responsible for cloudiness.
- Check the pressure on the filter gauge before and after backwashing: a persistent gap indicates a saturated filter media
- Replace the sand every five to seven years, or switch to filter glass which lasts longer and captures finer particles
- Clean the skimmer basket and the pump pre-filter at least once a week during peak season
Metal Oxidation: Copper and Iron, Hidden Enemies of Blue Water
A shimmering metallic blue water is not healthy water. It indicates the presence of oxidized copper, often introduced by a copper sulfate-based algaecide or by filling water rich in metals.
Copper oxidizes when the pH exceeds 7.6, and metallic reflections then appear quickly. Iron, on the other hand, gives brown or rusty hues that deposit on the liner and walls.
Protocol for Treating Metals in Pool Water
The metal sequestrant remains the reference product. It encapsulates metal ions to make them filterable. The effectiveness of the treatment depends on the order of operations:
- Temporarily raise the pH between 7.5 and 7.8 to promote copper precipitation
- Add a flocculant (cartridge or liquid) in the skimmer if the filter is sand
- Allow filtration to run for 24 hours without interruption, then perform a backwash
- Readjust the pH between 7.2 and 7.4 once the water is clear again
For iron-rich water (well water), a preventive treatment with a sequestrant at each filling prevents discoloration from the start. Testing the metal content of the filling water with a specific kit allows for dose calibration.
Regular Maintenance and Mistakes That Cause Loss of Blue Color
Shock chlorine is not a regular maintenance product. Using it every week destabilizes the chemical balance and accelerates the degradation of the liner. A regular treatment with slow chlorine is sufficient in the vast majority of cases.
Overdosing with copper algaecide is the most common mistake. Many owners double the preventive dose thinking they are better protecting the pool, while they introduce an excess of copper that will eventually oxidize and color the water.
Weekly Checkpoints
Three actions are enough to maintain water clarity throughout the season. Measuring pH, disinfectant levels, and filter pressure once a week allows for detecting any drift before it becomes visible. Adjusting filtration time as soon as the water temperature rises remains the most effective reflex.
Regular mechanical maintenance (brushing the walls, vacuuming the bottom, cleaning the waterline) complements chemical treatment. Unremoved organic matter consumes free chlorine and feeds algae before the disinfectant can act.
The blue color of a pool is not a given: it is the visible result of a continuously maintained chemical and mechanical balance. Monitoring the trio pH-TAC-TH, adapting filtration to temperature, and controlling metal inputs covers almost all situations of drift.